For simplicity and ease of construction, the typical alternating current generator, or alternator, includes a stationary armature winding composed of a large number of individual conductors assembled in slots formed in the inner surface of a hollow cylindrical iron stator, and a revolving field structure, or rotor, which has a plurality of individual field windings and is mounted for rotation within the stator. When the armature winding is installed, the conductors are connected in pairs to form coils which are so positioned in the stator that the two conductors of each coil overlie field windings of opposite polarity. The coils are in turn connected in a group or groups, one such group for each phase of the alternator. Since relative movement between a magnetic field and a sequence of conductors is necessary for production of electricity, the field windings of an alternator must be energized or excited. Most generators are synchronous machines that are designed to enable them to supply their own magnetic requirements. This is accomplished by applying DC power through brushes and slip rings or, in a brushless synchronous unit, by an inductive coupling to a secondary (DC) generator on the same shaft.
A less expensive alternative is to use a standard induction motor and drive it with another power source, i.e. a combustion engine, to generate electricity. Such units have to rely on a host utility to create the magnetic field. If that power source is cut off, the induction generator will cease production of electricity. Thus, all conventional induction machines are dependent upon an outside power source for their magnetic requirements.
A basic requirement for an induction alternator is that a revolving magnetic field must be produced in the air gap between the rotor and the stator. In a two-phase or any polyphase induction alternator, the fact that the currents flowing in the different phase windings are at 90 electrical degrees to each other produces a sinusoidally distributed magnetic field which revolves in synchronism with the magnetized rotor field. In the most common types of conventional alternators, the magnetic field has typically been energized by current supplied from a source which is external of the alternator itself. This is particularly true in the case of single-phase asynchronous alternators wherein the pulsating stator field produced is non-directional and does not create a revolving field. Without the influence of an out-of-phase or reactive current, the magnetic field created in the gap between the armature winding and the rotating field windings in a single phase asynchronous alternator will alternately expand and collapse. However, since there is no movement of the magnetic field between field windings the current thus generated is non-directional. In the most commonly used single phase asynchronous generators, i.e. split capacitor alternators, a large energy winding is directly connected to the power supply line and an out-of-phase current is supplied by a smaller auxiliary winding and a serially connected capacitor which are connected to the energy winding and across the power line.
Even the split capacitor alternators are efficient only when the magnetic field in the large energy winding is balanced with that of the auxiliary winding and their respective currents are displaced by 90 electrical degrees. Since the 90 degree displacement exists only at design load, a disproportionate distribution of magnetic flux occurs at other load points, with consequent negative sequence currents in the rotor and stator, space harmonics in the air gap, and high leakage reactance. Furthermore, the energy produced by the collapse of the magnetic field is returned to the system as VARS which adversely affects power factor and efficiency. Accordingly, asynchronous single-phase alternators have achieved only limited acceptance in industry due to the fact that they typically operate with efficiencies of 40-60% and power factors of 10-60%. In addition, because the LC circuit is tied directly to the power line, whenever a split capacitor alternator is connected to a non-linear load there is the risk of drawing high current to the resonant winding to the point of failure of the alternator.